The present invention relates generally to a communications system and, more specifically, to an apparatus and method for implementing a communications interface protocol using existing Cellular Digital Packet Data (xe2x80x9cCDPDxe2x80x9d) transmission equipment to implement point-to-point communications. More specifically, the invention relates to an apparatus and method of monitoring data collected by instrumentation such as temperature, pressure and flow meters located at sites remote from the process control computer, display or programmable logic controller to which the information is to be transmitted.
Collecting and monitoring data from a variety of locations for input into a central processing unit is a required function in a wide variety of applications. For example, in a chemical processing plant, such as a refinery, a variety of process parameters such as temperatures, pressures and flows are continuously or intermittently monitored and recorded. Similarly, in oil and gas production parameters such as pressures, flows and valve positions are measured and monitored. Typically, these parameters are measured at different locations in the particular process, and in some cases the instrumentation may be located at a remote site. Measurements are normally collected by the instrumentation as analog signal that is converted to a digital format and subsequently transmitted to a computing device such as a programmable process controller or process control computer via dedicated hard wired serial data interfaces. However, in the case where the instrumentation is located at a remote site, it may be difficult or cost prohibitive to provide the direct hard wired connection between the serial data interface at the particular instrument""s location and the interface at the particular computing device. On the other hand, wireless transmission of digital data over long distances has heretofore proven unreliable.
Cellular Digital Packet Data (xe2x80x9cCDPDxe2x80x9d) is a wireless communications protocol that folds streams of data into envelopes or packets that are transmitted at very high speeds during pauses in cellular phone conversations. This permits the use of existing cellular systems as a means of data transmission. CDPD allows data files to be assembled into packets and transmitted via idle channels of existing, but CDPD-upgraded, cellular voice networks. CDPD allows the transmission of data at 19.2 Kbps over an enhanced cellular network. Adding CDPD to an existing analog cellular system allows cellular systems to transmit data eight times faster without the necessity of creating a completely new digital system.
In practice, packet data is transmitted in a wireless mode using bandwidth of the Advanced Mobile Phone Service (xe2x80x9cAMPSxe2x80x9d) which operates as the communications infrastructure for analog cellular radio. Digital cellular is referred to as D-AMPS. CDPD specifications are published through the CDPD Forum and follow OSI (Open Systems Interconnection Protocol) guidelines. CDPD technology provides connectivity up to the network layer and is an overlay system that operates on AMPS frequencies.
The RS232 serial interface provides a serial data connection between two devices over dedicated wires. The interface defines up to 26 lines between two devices. One line carries the data and the others carry signaling information. Signaling is achieved by the lines through binary states, either xe2x80x9cONxe2x80x9d or xe2x80x9cOFF.xe2x80x9d Some lines are defined for data and some for signaling. Thus, data transmission can be controlled simultaneous by both the sending device and the receiving device.
For example, the sending device can query the receiving device as to whether it is ready to receive data by setting the signal high on a line called Ready To Send (xe2x80x9cRTSxe2x80x9d). The receiving device can, in turn, reply that it is ready to receive data by setting the signal high on the Clear To Send (xe2x80x9cCTSxe2x80x9d) line. After these conditions are satisfied, by devices can begin data transmission over the data lines a communication speeds.
The present invention provides a system for point-to-point communication of digital data between devices utilizing RS232 interfaces. In one embodiment, process data, such as temperatures, pressures and flows are monitored at remote locations. The data is converted from an analog signal to a digital format and transmitted to a wireless modem via an RS232 serial data interface associated with the particular instrumentation. The data is then transmitted from the first wireless modem to a second wireless modem. The second wireless modem communicates the information to a device such as a process control computer or programable logic controller.
Wired RS232 applications conduct data communications between a sender and a receiver that are hard wired together. In a wired RS232 connection based application, several lines may simultaneously used to send signaling information; the signals on each line are detected by the sender and receiver instantaneously. On the other hand, in wireless RS232 communications, instantaneous communication does not occur. Rather, information is transmitted with a delay of ranging from 0.5 to 4 or 5 seconds. Furthermore, in wireless communications, modems can connect to a large number of devices using the call establishment methodology programmed into the modems.
The present invention provides a method and apparatus for using wireless modems for point-to point transmission of serial data in an RS232 mode, that provide (1) logic establishing the sender and receiver and (2) a signaling and data transmission methodology that makes the wireless mode transparent to the applications. The method and apparatus of the present invention allows utilization of wired RS232 based applications transparently on wireless communication systems, and alleviates the delay inherent in wireless communications without impacting the reliability of the system.
The invention provides a wireless point-to-point communications system for reliable and efficient digital data transfers as compared to prior art network interface protocols. In this regard, and in accordance with one embodiment, the invention utilizes a commonly available communications protocol, such as RS232, or other accepted serial standard to encapsulate digital data derived from an instrument, data collector, or other signal acquisition means in a wireless signal carrier. The serial protocol has built-in error correction and flow control. The end-to-end interface is compatible with ordinary devices supporting the protocol. In addition, the cellular communications infrastructure provides a reliable backbone with call switching and routing of the data to its intended destination. The RS232 data and flow control signals are encapsulated in a CDPD carrier supported in existing cellular infrastructure. Since RS232 is widely accepted and supported by a plurality of data acquisition and processing systems, the interface protocol provided is reliable and efficient.
The present invention is particularly adaptable to systems incorporating remote data collection devices such as process control systems, environmental monitoring systems, pipeline monitoring systems.